The major career goals of the applicant are to elucidate the role of exercise on the development of bone strength during adolescence and young adulthood. Fractures occur when bone strength, a product of bone material and structural properties (bone size and architecture) either decreases (as in osteoporosis) or is exceeded (as in trauma). Progress in this field has become limited because the most advanced clinical tool for bone, dual energy x-ray absorptiometry (DXA), does not assess bone geometry and structure, important contributors to bone strength. However, the needed bone geometry information can be acquired by magnetic resonance imaging (MRI) and software algorithms applied to DXA data. Research plan. Two experiments will be conducted to 1) establish the in vivo accuracy of magnetic resonance imaging (MRI) and DXA for assessing bone geometry and bending strength and 2) determine the effect of adolescent physical activity on young adult bone geometry using two novel techniques (MRI and DXA structure analysis). Didactic Training. The educational program will be overseen by 3 co-mentors: Tim Mosher, M.D., will be responsible for all aspects related to MRI: Tom Beck, ScD, will be responsible for all aspects related to DXA-derived structural analyses; and Tom Lloyd, Ph.D., will be responsible for all aspects related to protection of human subjects, data analysis, manuscript and grant preparation. Anticipated results. The proposed research and educational program will establish new methods for measuring bone geometry, contribute to our understanding of the role of exercise during childhood on adult bone strength, and enable the candidate to establish an independent research program using novel methods of assessing bone geometry and muscle mass.